Move text message handling into the new plugin system

src/plugins/TextMessagePlugin.h

@@ -0,0 +1,28 @@
+#include "MeshPlugin.h"
+#include "Observer.h"
+
+/**
+ * Text message handling for meshtastic - draws on the OLED display the most recent received message
+ */
+class TextMessagePlugin : public MeshPlugin, public Observable<const MeshPacket *>
+{
+  public:
+
+    /** Constructor
+     * name is for debugging output
+     */
+    TextMessagePlugin() : MeshPlugin("text") {}
+
+    /**
+     * @return true if you want to receive the specified portnum
+     */
+    virtual bool wantPortnum(PortNum p) { return p == PortNum_TEXT_MESSAGE_APP; }
+
+    /** Called to handle a particular incoming message
+
+    @return true if you've guaranteed you've handled this message and no other handlers should be considered for it
+    */
+    virtual bool handleReceived(const MeshPacket &mp);
+};
+
+extern TextMessagePlugin textMessagePlugin;